Mutations in the recF gene of E. coli cause a reduction in the expression of the SOS response, UV sensitivity, and a reduction in the level of induced recA protein synthesis. Increasing the levels of recA protein synthesis, either by introducing a multicopy plasmid bearing the recA gene or introducing a recA operator mutation, has no effect on the recF phenotype. Introduction of the recA441 mutation however, results in complex phenotypic changes which yield new insights into the physiological effects of recF mutations. The recA441 allele is unique since, unlike other recA mutations, it causes the expression of the SOS response without DNA damage upon incubation at 42C.